The present invention relates to a steering control device equipped with a turning mechanism including a turning actuator driving a turning shaft whose position can be controlled and a steering angle sensor detecting a steering angle θ of a steering wheel.
The present invention is useful in various types of automotive steering control devices such as “steer-by-wire” systems and “variable gear ratio” systems.
Widely known examples of standard, conventional steer-by-wire systems include the technologies in Japanese patent publication number 2001-334947 and Japanese patent publication number Hei 5-105100 below.
FIG. 24 is a control block diagram illustrating the control system of a steering control device 900, which is a conventional steer-by-wire system.
The steering mechanism of the steering control device 900 is formed primarily from a steering shaft separated from a turning shaft 8, a steering wheel 1, a torque sensor 3 (steering torque sensor), a reaction motor 4 (steering actuator), a reaction control section 5, and the like.
The reaction control section 5 and the position control section 10A can be formed as separate processing devices (control devices) or as two control programs executed by a single processing device (control device). Also, the motor driver circuits (not shown in the figure) driving the motors 4, 6 can be installed at their respective motors or can be installed at the processing device (control device). These aspects of the structure can generally be designed freely.
The reaction control section 5 determines an instruction current in for the reaction motor 4 based on a steering torque τ output by the torque sensor 3 and an instruction current In for the turning motor 6, which is determined by a predetermined feedback control (hereinafter referred to as a position control A) executed by the position control section 10A.
Also, the turning mechanism of the steering control device 900 is formed primarily from the position control section 10A, which executes the position control A described above, the steering angle sensor 2, the turning motor 6 (turning actuator), the position sensor 7 (turning displacement sensor), the turning shaft 8, the tires 9, and the like.
FIG. 25 is a control block diagram illustrating the control system of the position control section 10A of the conventional steering control device 900 described above. A turning instruction value calculating section 11A for executing the position control A determines an instruction value Xn for the turning displacement of the turning shaft so that the value is roughly proportional to the steering angle θ. A PID control section 12 uses widely known PID control operations to determine an instruction current In for the turning motor 6 based on the turning displacement instruction value Xn and a turning displacement measurement value Xa. With this position control A, the orientation of the tires 9 is controlled so that a desired orientation is achieved.
In the control system described above, a large steering operation that causes the turning displacement instruction value Xn to exceed an actual physical end position (+/−XE), the instruction current In will increase suddenly based on this excess. Since the action of the reaction control section 5 will lead to a sudden increase in the output torque (steering reaction) of the reaction motor 4 as well, there will naturally be a generated (simulated or emulated) virtual endpoint for the steering range even if there is no physical restriction (endpoint or abutment position) to the rotation range of the steering wheel 1.
In other words, in cases such as when there is no physical restriction (endpoint or abutment point) to the rotation range of the steering wheel 1, the conventional control system described above is useful in generating a virtual abutment counterforce (steering reaction) that prevents a steering angle θ from exceeding a threshold tolerance range (−θE≦θ≦θE).
However, in the conventional system described above, the virtual abutment counterforce (steering reaction) near the endpoints of a predetermined steering range is a force created by the increase of the instruction value Xn in tandem with θ while the turning displacement Xa is mechanically fixed at the end. When this counterforce is generated, the instruction current In for the turning motor 6 can become very high. Thus, if this state continues for an extended period, the turning motor 6 may overheat and malfunction.
Thus, the conventional system described above can lead to obstacles to providing a compact and light design for the turning actuator (turning motor 6). As a result, the use of this conventional system will be inefficient in terms of production costs for automobiles, the degree of freedom made available in automobile design, performance of automobiles, and the like.
FIG. 1 is a control block diagram illustrating a control system of a steering control device 100 that includes a heating prevention function and that was developed to overcome the overheating problem described above. In this steering control device 100, the position control system of the turning shaft 8 is somewhat different from that of the position control section 10A of the steering control device 900 described above.
A position control section 10B shown in FIG. 1 includes a turning instruction value calculating section 11B shown in FIG. 2 in place of the turning instruction value calculating section 11A shown in FIG. 25. The position control (position control B) of the turning shaft 8 is implemented by the actions of the turning instruction calculating section 11B and the PID control section 12. The other elements such as the PID control section 12 are the same as described above.
FIG. 2 is a graph illustrating the calculations performed by the turning instruction value calculating section 11B of the steering control device 100. In this graph, +/−XE represents the tolerance range for turning displacement and is set according to the actual limit points for turning shaft displacement. If, for example, upper and lower limits for the turning displacement instruction value Xn are guarded in this manner with a limiter or the like, excessive values for the instruction current In for the turning motor 6 would be prevented, thus eliminating the heating problem described above.
However, when this type of guarding procedure is implemented, the actions of the PID control section 12 and the counterforce control section 5 will also restrict the output torque (turning counterforce) of the reaction motor 4, thus preventing the virtual steering range endpoints from being generated (simulated or emulated) as in the steering control device 900 shown in FIG. 24.
Also, since no abutment counterforce (steering counterforce) will be generated as in the steering control device 900, the steering wheel will be able to easily enter the regions indicated by the shading in FIG. 2. Thus, when the steering angle θ enters this “play” region, linear steering feel and turning responsiveness are decreased.
The object of the present invention is to overcome the problems described above. When the steering angle θ exceeds a tolerance range (−θE≦θ≦θE), a localized gear ratio (∂Xn/∂θ) is prevented from being continuously 0 during the subsequent return steering. This maintains steering responsiveness during this return steering.
Means for overcoming the above problems will be described.
According to first means of the present invention: in a steering control device equipped with: a turning mechanism including a turning actuator driving a position-controllable turning shaft; and a steering angle sensor detecting a steering angle θ of a steering wheel, a steering control device includes: a turning displacement sensor detecting a turning displacement X (−XE≦X≦+XE) in the turning mechanism; and means for calculating a turning instruction value calculating an instruction value for a turning displacement in the turning mechanism based on the steering angle θ. The turning instruction value calculating means includes means for generating hysteresis characteristics calculating, in exceptional situations where an absolute value |θ| of the steering angle θ exceeds a predetermined threshold value θE corresponding to an upper limit XE of the turning displacement X, the instruction value Xn based on: a vertical axis coordinate corresponding to the steering angle θ on a predetermined hysteresis loop with one side being a section of a line Xn=+−XE on a θ−Xn plane; and a steering direction (steering direction/restoring direction) of the steering wheel.
The “line Xn=+/−XE” referred to above indicates either a line Xn=+XE or a line Xn=−XE. More specifically, when steering takes place to the left and steering exceeds the left-side endpoint, the former (line Xn=+XE) applies. When steering takes place to the right and steering exceeds the right-side endpoint, the latter (line Xn=−XE) applies. Also, the use of the “+−” expression below should be interpreted in this manner unless otherwise stated.
With the structure described above, if the steering angle θ suddenly enters a “play” region at the left or right end as shown in FIG. 2, a localized gear ratio (∂Xn/∂θ) can be changed according to the direction in which the steering wheel is turned (turning or restoring direction). FIG. 3 shows a graph illustrating an example of how this type of hysteresis loop is formed.
More specifically, when the driver turns the steering wheel in the restoring direction to come out of the “play” region, the gear ratio (∂Xn/∂θ) can be kept at a positive value even in the “play” region with the hysteresis loop described above.
Thus, with this structure, if the steering wheel is turned so that the tolerance range of the steering angle θ is exceeded (−θE≦θ≦θE), the localized gear ratio (∂Xn/∂θ) is prevented from being continuously 0 in the restorative steering that takes place immediately afterwards. As a result, steering responsiveness can be maintained during this restoring operation.
More specifically, with the present invention, a turning displacement that is roughly proportional to the steering amount can be immediately obtained when restoring the steering wheel, even if the steering status is within a “play” region shown in FIG. 2. Thus, even if the steering status is in this type of “play” region, safety based on linear steerability can be maintained.
Also, with the structure described above, a linear steering “feel” can be provided when the steering wheel is being restored, even if the steering angle θ suddenly enters a “play” region.
According to second means of the present invention, a steering mechanism including the steering wheel and the turning mechanism are mechanically separated, and an electrical coupling mechanism substitutes for a connecting mechanism connecting the steering mechanism and the turning mechanism.
More specifically, in a “steer-by-wire” system, the present invention provides extremely desirable advantages. In a steer-by-wire system that separates a steering mechanism including the steering wheel from the turning mechanism and that substitutes an electrical coupling mechanism for a connecting mechanism connecting the steering mechanism and the turning mechanism, the need to dynamically vary the gear ratio (∂X/∂θ) according to the automobile velocity or the like makes it necessary to also vary the effective steering range of the steering wheel (the steering angle range where the gear ratio is positive), i.e., the tolerance range (−θE≦θ≦θE) of the steering angle θ. As a result, “play” regions will necessarily be generated outside of the tolerance range of the steering angle θ at least when the gear ratio is large.
Also, with standard steer-by-wire system steering wheels, endpoints for the rotation range, i.e., “ends” of the steering range where steering operations is blocked, are often not physically installed due to these factors. As a result, the present invention provides major advantages in steering control devices that involve these factors.
Third means of the present invention is as described in first or second means and further includes means for setting an endpoint setting a target coordinate for an endpoint PO that closes the hysteresis loop.
When the endpoint PO of the hysteresis loop reaches the opposite side past the origin (the diagonal quadrant), a situation develops where when the steering angle θ becomes 0 on the hysteresis loop, the turning displacement instruction value Xn does not become 0. This means that the neutral point of the steering wheel is misaligned, and is an undesirable situation.
Also, as the hysteresis loop endpoint PO approaches the steering wheel rotation range endpoints (+/−θE), the effect of maintaining steering responsiveness gradually diminishes, as can be seen from FIG. 3. More specifically, as the endpoint PO approaches the entry point of the “play” region, the gear ratio (∂X/∂θ) when leaving the “play” region monotonically approaches 0, making it undesirable for the endpoint PO to be set up in the vicinity of the “play” region entry point. Also, the value of the gear ratio (∂X/∂θ) is closely associated with steering “feel”, so it would be preferable to restore the relation between θ and Xn to a normal state as quickly as possible when high responsiveness is not required. In this sense, therefore, a smaller hysteresis loop is better.
As described above, for example, suitable or optimal coordinates for the endpoint PO need to meet many conditions, and there are not a few conflicting trade-offs associated with these coordinates. Thus, in order to provide good steering responsiveness and “feel”, it is extremely important to empirically determine endpoint PO coordinates that rationally meet these various conditions simultaneously.
With endpoint setting means described above, these types of suitable or optimal endpoint PO coordinates can be set up in the steering control device. This greatly assists in providing good steering responsiveness and “feel”.
Fourth means of the present invention is as described in third means wherein the endpoint setting means includes means for varying a target point dynamically varying the target coordinate for the endpoint PO based on a steering velocity ω (=dθ/dt), a steering torque τ, the steering angle θ, the steering direction, or an automobile velocity v.
When a steering wheel restore operation is taking place in order to leave the “play” area, it is preferable to have a high gear ratio (∂X/∂θ) in cases of sudden avoidance operations. Also, in normal conditions where special suddenness is not involved, it would be preferable to restore the relation between θ and Xn to the normal state as quickly as possible.
For example, the ideal coordinates of the endpoint PO on the θ−Xn change according to the steering status. Since the steering status can be dynamically estimated based on steering velocity ω (=dθ/dt), steering torque τ, steering angle θ, steering direction, or the automobile velocity v, target point varying means described above can allow the target coordinates of the endpoint PO to be dynamically optimized for different steering status conditions.
Fifth means of the present invention is as described in any one of first means through fourth means wherein, in the hysteresis loop, all points except an origin on a horizontal axis of the θ−Xn plane are outside the hysteresis loop.
As mentioned before, when the endpoint PO of the hysteresis loop passes the origin and reaches the opposite side (diagonal quadrant), the turning displacement instruction value Xn will not be 0 when the steering angle θ on the hysteresis loop is 0. This leads to the neutral point of the steering wheel being misaligned and is not desirable. This type of problem can be avoided, however, if the hysteresis loop is formed so that all points on the horizontal axis of the θ−Xn plane except for the origin are outside of the hysteresis loop.
However, if a dead zone (⊂ horizontal axis) near the origin, i.e., a non-responsive region where Xn=0, ∂Xn∂θ=0, is to be set up, the problem described above can be avoided by arranging the endpoint PO of the hysteresis loop at the endpoint (∈ horizontal axis) of the dead zone.
Sixth means of the present invention is as described in fifth means wherein, using a function f(θ) of the steering angle θ, symmetrical around the origin, and a correction gain G (0<G≦1), the hysteresis loop on the θ−Xn plane is a closed curve formed from the line Xn=+/−XE, a curve Xn=f(θ), and a curve Xn=Gf(θ).
The curve Xn=f(θ), however, can be formed as a line or from line segments. The same goes for curve Xn=Gf(θ). Also, the function f can be defined as an equation or can be implemented with a map (table data) and interpolation operations.
If the function y=f(θ) is a monotonically increasing function that is symmetrical around the origin and its domain is made adequately wide, the value of the correction gain G described above can be selected appropriately so that the hysteresis loop can be formed with the loop endpoint PO on the origin as described above. Also, the path, size, and the shape of the hysteresis loop can be selected continuously and freely using the correction gain G described above as a parameter.
More specifically, with the arrangement above, a small number of control parameters can be used to define or control the path, size, and shape of the hysteresis loop on the θ−Xn plane.
Seventh means of the present invention is as described in sixth means and further includes means for calculating correction gain calculating a value for the correction gain G based on the upper limit XE and the function f(θ).
For example, the equation of a line that overlaps with path (a) in FIG. 3 is assumed to be y=f(θ)=aθ. Then, by setting the turning point in the “play” region in FIG. 3, i.e., the horizontal axis coordinate of the starting point of path (c), to θR and the correction gain value to G=XE/|f(θR)|, the curve Xn=Gf(θ) described above becomes a line that connects the starting point of the turning point and the origin. In other words, in this case the endpoint PO of the hysteresis loop matches the origin.
More generally, the function y=f(θ) does not necessarily have to be a line. The only conditions that must be met are that it be a monotonically increasing function that is symmetrical around the origin and that its domain is adequately wide, as described above. Thus, correction gain calculating means described above is extremely useful in determining a turning point path for the most simple and basic hysteresis loop.
Eighth means of the present invention is as described above in sixth means or seventh means wherein the function f(θ) is a quadratic equation of the steering angle θ.
For example, if the function f is set up as a quadratic equation of θ, as in “y=f(θ)=a(|θ|+b)θ, a>0, b>0”,then the gear ratio (∂X/∂θ) is relatively limited in the vicinity of the neutral point. This makes it possible to provide a relatively stable steering “feel” in the vicinity of the neutral point. The function f can, for example, be determined in this manner.
Ninth means of the present invention is as described in any one of sixth means through eighth means wherein the function f(θ) can be expressed as “f(θ, v)=θ∘g(θ, v)” where the function g(θ, v) is a function of automobile velocity v or the steering angle θ and has as a factor the steering angle θ.
Setting up the function f in this manner is convenient because it always passes the origin. Also, by making the function f dependent on the automobile velocity v and by setting up this dependency in an appropriate manner, it is possible to meet various demands, e.g., dynamically varying the gear ratio (∂X/∂θ) according to the automobile velocity.
Tenth means of the present invention is as described in any one of sixth means through ninth means further including means for asymptote normalization monotonically increasing the correction gain G(0<G≦1) in a dynamic manner based on a steering amount S, a steering status, a turning amount Z, or a turning status after initiation of restorative steering having as a starting point the line Xn=+/−XE.
With this type of arrangement, the size, the turning-point path, and the shape of the hysteresis loop on the θ−Xn plane can be dynamically and freely changed. Also, the position of the endpoint PO of the hysteresis loop can be dynamically and freely changed as well. However, if the value of the correction gain G is to be changed, it would be preferable in terms of generating a natural steering “feel” to change the correction gain G in a monotonic or continuous manner as much as possible.
For example, if, as mentioned previously, the equation of a line that overlaps with the path (a) in FIG. 3 is set to y=f(θ)=aθ, the horizontal coordinate of the turning point in the “play” region in FIG. 3, i.e., starting point of path (c), is set to θR, and the correction gain value is set to G=XE/|f(θR)|, the curve Xn=Gf(θ) becomes a line connecting the starting point of the restoring and the origin. Thus, in this case, the equation of the restoring path (c) is provided by the line Xn=Gf(θ), and the endpoint PO of the hysteresis loop matches the origin.
In this type of situation, monotonically increasing the correction gain value G=XE/|f(θR)| to gradually approach 1 as the point (θ, Xn) on the hysteresis loop moves along the restoring path (c) results in the restoring path (c) becoming a downwardly projecting curve and the endpoint PO shifting to the right of the origin as the correction gain G dynamically increases. If θ<0, the same applies except everything is converted to be symmetrical around the origin. For example, if the hysteresis loop is to be made small, the correction gain G can be made to approach 1 as the displacement along the restoring path (c) (or the steering amount S described above) increases. The more quickly the correction gain G converges to 1, the smaller the hysteresis loop will be.
Thus, for example, with this type of means, the path, size, and shape of the hysteresis loop on the θ−Xn plane can be dynamically and freely controlled with a small number of control parameters.
Eleventh means of the present invention is as described in tenth means wherein the asymptote normalizing means includes means for varying an asymptote rate using a steering velocity ω (=dθ/dt), a steering torque τ, the steering angle θ, the steering direction, or an automobile velocity v, in order to dynamically change an asymptote rate A (≡dG/dS) for the steering amount S of the correction gain G or an asymptote rate B (≡dG/dZ) for the turning amount Z of the correction gain G when the correction gain G (0<G≦1) is being monotonically increased in a dynamic manner.
As the observations above indicate, the hysteresis loop can be made smaller the larger the asymptotic rate A(≡dG/dS) is. Thus, by changing this value dynamically, the shape, the position, and the length of the restoring path (c) can be changed dynamically. Or, the position of the endpoint PO can be dynamically optimized. However, this requires caution since making the asymptotic rate A too big can result in a negative value for the gear ratio (∂X/∂θ).
These issues also apply when the rate of increase of the correction gain G is controlled using the asymptotic rate B(≡dG/dZ) instead of the asymptotic rate A(≡dG/dZ).
If, as a result of some operation, the gear ratio (∂X/∂θ) can become a negative value, it would be preferable to handle these situations by optimizing the length of the control interval or by performing smoothing operations or the like. Also, temporary (instantaneous) negative values in the gear ratio (∂X/∂θ) caused by fine vibrations in the automobile or the reaction motor or the like can sometimes be prevented using standard, widely known noise-handling techniques or the like.
Twelfth means is as described in any one of first means through eleventh means further including means for varying a steering angle threshold dynamically changing upper and lower limits of a predetermined tolerance range (−θE≦θ≦θE) of the steering angle θ based on an automobile velocity v.
The operations and advantages of the present invention according to this aspect will be described in detail in the section on the fourth embodiment below.
Thirteenth means is as described in any one of first means through twelfth means wherein the steering mechanism includes means for generating endpoint reactions generating, at a vicinity of an upper limit position θE of the steering angle θ and at a vicinity of a lower limit position −θE of the steering angle θ, a virtual abutment resistance restricting the steering angle θ from exceeding a predetermined tolerance range (−θR≦θ≦θE), based on the steering angle θ, the turning displacement X, or an instruction value Xn for the turning displacement X.
The operations and advantages of the present invention according to this aspect will be described in detail in the section on the fourth embodiment below.
With these aspects of the present invention, the problems described above can be effectively or rationally overcome.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.